CS211: Fall 2024
Defusing a Binary Bomb
Due: December 15th, 2024
CS211 Sections 5 - 8
1 Introduction
The nefarious Dr. Evil has planted a slew of “binary bombs” on our class machines. A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on stdin. If you type the correct string, then the phase is defused and the bomb proceeds to the next phase. Otherwise, the bomb explodes by printing "BOOM!!!" and then terminating. The bomb is defused when every phase has been defused.
There are too many bombs for us to deal with, so we are giving each student a bomb to defuse. Your mission, which you have no choice but to accept, is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!
Step 1: Get Your Bomb
Due to firewall restrictions, to obtain your bomb, you must first loginto aniLab machine using weblogin. Next, open a web browser on any iLab machine you choose from the WebLogin list. Navigate to:
http://h418-1 .cs .rutgers .edu:17220
This will display a binary bomb request form for you to fill in. Enter your netid and email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar file called bombk .tar, where k is the unique number of your bomb.
Save the bombk .tar file to a (protected) directory in which you plan to do your work. Then give the command: tar -xvf bombk .tar. This will create a directory called ./bombk with the following files:
• README: Identifies the bomb and its owners.
• bomb: The executable binary bomb.
• bomb .c: Source file with the bomb’s main routine and a friendly greeting from Dr. Evil.
• writeup.{pdf,ps}: The lab writeup.
If for some reason you request multiple bombs, this is not a problem. Choose one bomb to work on and delete the rest.
Step 2: Defuse Your Bomb
Your job for this lab is to defuse your bomb.
You must do the assignment on one of the class machines. In fact, there is a rumor that Dr. Evil really is evil, and the bomb will always blow up if run elsewhere. There are several other tamper-proofing devices built into the bomb as well, or so we hear.
You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best way is to use your favorite debugger to step through the disassembled binary.
Each time your bomb explodes it notifies the bomblab server, and you lose 1 point in the final score for the lab. So there are consequences to exploding the bomb. You must be careful!
Each Phase will be 7 points. the maximum you maybe able to get is 102.
Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty. However, the last phase will challenge even the best students, so please don’t wait until the last minute to start.
The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,
linux> ./bomb psol .txt
then it will read the input lines from psol .txt until it reaches EOF (end of file), and then switch over to stdin. In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping the solutions to phases you have already defused.
To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code and how to set breakpoints. You will also need to learn how to inspect both the registers and the memory states. One of the nice side-effects of doing the lab is that you will get very good at using a debugger. This is a crucial skill that will pay big dividends the rest of your career.
Logistics
This is an individual project. All handins are electronic. Clarifications and corrections will be posted on the course message board.
Scoreboard
You can keep track of how you are doing by looking at the class scoreboard at:
http://h418-1 .cs .rutgers .edu:17220/scoreboard
This webpage is updated continuously to show the progress for each bomb.
Hints (Please read this!)
There are many ways of defusing your bomb. You can examine it in great detail without ever running the program, and figure out exactly what it does. This is a useful technique, but it not always easy to do. You can also run it under a debugger, watch what it does step by step, and use this information to defuse it. This is probably the fastest way of defusing it.
We do make one request, please do not use brute force! You could write a program that will try every possible key to find the right one. But this is no good for several reasons:
• You lose 1 point every time you guess incorrectly and the bomb explodes.
• Everytime you guess wrong, a message is sent to the bomblab server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer access.
• We haven’t told you how long the strings are, nor have we told you what characters are in them. Even if you made the (incorrect) assumptions that they all are less than 80 characters long and only contain letters, then you will have 2680 guesses for each phase. This will take a very long time to run, and you will not get the answer before the assignment is due.
There are many tools which are designed to help you figure out both how programs work, and what is wrong when they don’t work. Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them.
• gdb
The GNU debugger, this is a command line debugger tool available on virtually every platform. You can trace through a program line by line, examine memory and registers, look at both the source code and assembly code (we are not giving you the source code for most of your bomb), set breakpoints, set memory watch points, and write scripts.
The CS:APP web site
http://csapp.cs.cmu.edu/public/students.html
has a very handy single-page gdb summary that you can print out and use as a reference. Here are some other tips for using gdb.
– To keep the bomb from blowing up every time you type in a wrong input, you’ll want to learn how to set breakpoints.
– For online documentation, type “help” at the gdb command prompt, or type “man gdb”, or “info gdb” at a Unix prompt. Some people also like to run gdb under gdb-mode in emacs.
• objdump -t
This will print out the bomb’s symbol table. The symbol table includes the names of all functions and global variables in the bomb, the names of all the functions the bomb calls, and their addresses. You may learn something by looking at the function names!
• objdump -d
Use this to disassemble all of the code in the bomb. You can also just look at individual functions. Reading the assembler code can tell you how the bomb works.
Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls to system-level functions are displayed in a cryptic form. For example, a call to sscanf might appear as:
8048c36: e8 99 fc ff ff call 80488d4 < init+0x1a0>
To determine that the call was to sscanf, you would need to disassemble within gdb.
• strings
This utility will display the printable strings in your bomb.
Looking for a particular tool? How about documentation? Don’t forget, the commands apropos, man, and info are your friends. In particular, man ascii might come in useful. info gas will give you more than you ever wanted to know about the GNU Assembler. Also, the web may also be a treasure trove of information. If you get stumped, feel free to ask your instructor for help.
submission
You must compile the phase inputs into a single new-line delimited file called mysolution.txt. You can validate that this file is accurate by running the command stated in Step 2. You have to submit the assignment using Canvas. To create the tar file that you will submit after finishing your programming assignment, you will use the following command line, in the parent directory of bomb[ID]:
$ tar -cvf bomb[id] .tar bomb[id]
your tar should contain the folder, bomb[id], which contains the same files that were downloaded plus your solutions file. To verify, you can run the following command. If you get a similar output, you’re fine:
ajf277@assembly:˜/Documents/CS211/testbomb$ tar -tvf bomb1 .tar
drwxr-x--- ajf277/ajf277 0 2024-11-22 01:43 bomb1/
-rw-r----- ajf277/ajf277 71 2024-11-22 01:34 bomb1/README
-rw-r----- ajf277/ajf277 4069 2024-11-22 01:34 bomb1/bomb .c
-rwxr-x--- ajf277/ajf277 38600 2024-11-22 01:34 bomb1/bomb
-rw-r----- ajf277/ajf277 64 2024-11-22 01:41 bomb1/solution .txt
ajf277@assembly:˜/Documents/CS211/testbomb$
You should be able to pass in the solution.txt file into your bomb program, and pass all phases.
Frequently Asked Questions
Q. When I try running the bomb, it prints ”Initialization error: Running on an illegal host [2].” Has Dr. Evil defeated me?
A. No, this simply means that you’re not running the bomb on a valid machine. make sure you use one of
the computers in the CAVE, command center or meltdown lab.
Q. Is there any extra credit for solving the secret phase?
A. What secret phase? ;-)